1. Field of the Invention
The present invention relates to an image forming device including a fixing portion including a nip that is formed between a heat roller and a back-up roller pressurized by a pressurizing mechanism and holds and conveys a toner-bearing sheet while fixing toner thereto, a pressure switching portion for switching the pressurizing force of the pressing mechanism between a strong force and a weak force, and a sheet detecting portion for detecting a sheet conveyed to the fixing portion.
2. Description of the Related Art
Heat roller-type fixers provided in electrophotographic image forming devices each include a heat roller heated to a predetermined temperature by a heater mounted inside the heat roller and a back-up roller pressed into contact with the heat roller.
The heat roller and the back-up roller are pressurized by a pressurizing mechanism to form a nip that holds a sheet, thus conveying it while fixing toner thereto.
Since suitable nip pressure varies depending on the kind of sheet passed through the fixing portion, image forming devices configured to be able to change the nip pressure depending on the kind of sheet have been conventionally proposed.
For example, Japanese Unexamined Patent Publication No. 6-337612 describes an image forming device including a fixing portion configured to hold and convey a record material while fixing an unfixed image thereto with a heat roller and a back-up roller, characterized in that the fixing portion is user operable and has a lever switchable between a first position at which normal nip pressure is effected to the heat roller and the back-up roller and a second position at which nip pressure for special paper is effected to the rollers.
Such an image forming device enables an operator to switch the nip pressure to the heat roller and the back-up roller by handling the lever in accordance with the kind of sheet.
The image forming device described in the JP6-337612 publication involves laborious handling when checking the current nip pressure; the operator has to open the cover of the device and check the position of the lever.
Convenience for the operator improves when the image forming device includes an actuator that operates in tandem to the lever and a pressurization state sensor including a photosensor for detecting the posture of the actuator so that the state of the nip pressure is automatically displayed on a display portion or the like of the image forming device on the basis of an output signal from the sensor.
However, adding the sensor to the image forming device is not preferable in that the manufacturing cost increases.